Thermo-sensitive switch plug



D. J. SCHMITT THERMO-SENSITIVE SWITCH PLUG July 27, 1965 2 Sheets-Sheet1 Filed Dec. 28, 1962 III:

s9 INVENTOR. DONALD J. SCHMITT BY M 14/23 FIG. 3

ATTORNE YS July 27, 1965 D. J. SCHMITT 3,197,594

THERMO-SENSITIVE SWITCH PLUG Filed: Dec 28,. 1962 2 Sheets-Sheet 2 llFIG. 6

FIFG: 5. 2O

MIC FIG. 8

INVENTOR. DONALD J. SCHMITT ATTORNEYS United States Patent ce O-Disc,incorporated, Manstieid, (ihio, a corporation of Ghio nine Dec. as,1962, Ser. Ne. 247,9ss 4- Claims. ea. sea-teas This invention relates toan electrical fitting of the type employed in attaching an electric cordto a conventional outlet or source of power embodying an electriccircuit breaker or switch therein and more particularly to a connectorplug having a thermo-sensitive switch as an integral part thereof whichis responsive to excessive heat produced by current overloads forinterrupting the flow of current through the fitting.

Heretofore, it has been the practice to provide protective devices forelectric motors used with various types of tools and appliances to avoidthe effects of overheating resulting from current overloads. However,this practice was substantially confined to motors of a size in whichthe protective device could be easily or economically incorporated inthe motor or in close proximity to the windings thereof. Thus, suchprotective devices were used primarily with medium and larger sizedmotors. Considerable trouble has been experienced heretofore with motorsused in connection with small hand power tools and similar appliances.The improper use or abuse of such tools and appliances has contributedgreatly to the burning out of the motors and damage to essentialoperating elements of the devices. The relatively small size of themotors used in connection with such devices precluded the use of thecustomary protective devices. It was, therefore, desirable that suitableprotective means be developed and provided for motors used with powertools and other appliances where current overloads might be experienced.

It is among the objects of the present invention to provide a new andimproved means for overcoming the dirfi culties heretofore experiencedin protecting motors employed in operating small hand tools and similarappliances.

Another object of the present invention is to provide a device forprotecting electrical motors employed in operating small hand tools andsimilar appliances which causes interruption of the circuit only after asustained overload of sufficient duration effective to damage the motorwindings and avoids nuisance tripping of the circuit control means inresponse to overloads of short duration resulting from harmless currentsurges.

Another object of the present invention is to provide an electrical plugembodying a current overload responsive switch, which includes a thermalelement having two positions of concavity, which element is free to passautomatically from its first position of concavity to its secondposition upon a rise in temperature and is effective to open the switchcontacts in its second position and which may be returned to its firstposition only by manually operated reset means.

Another object of the invention is to provide cal plug embodying athermostatic switch having manually operable reset means therefor,wherein the switch mechanism is trip-free of the manual resetting means.

A still further object of the invention is to provide an electricalfitting having a thermo-sensitive switch which because of itssimplicity, economy of construction and reliability and efliciency inoperation is particularly well adapted as a protective device incontrolling electric circuits wherein the current is employed foroperating purposes in small hand power tools and appliances.

These and other objects and advantageous features of an electri-3,197,594 Patented July 27, 1965 the invention not at this time moreparticularly pointed out will become apparent as the nature of theinvention is better understood from the following detailed descriptiontaken in conjunction with the accompanying drawings wherein likereference characters denote corresponding parts and wherein:

FIGURE 1 is a plan view of the electrical fitting of the presentinvention showing the operating button of the manual reset meansprojecting through the housing of the fitting,

FIGURE 2 is a longitudinal sectional View taken on the line 22 of FIGURE1 illustrating the elements of the device,

FIGURE 3 is a transverse sectional view taken on the line 33 of FIGURE 2showing the contacts of the thermostatic switch in closed position,

FIGURE 4 is a transverse sectional View taken on the line 3-3 of FIGURE2 showing the contacts of the thermostatic switch in open position,

FIGURE 5 is a transverse sectional view taken on the line 3-3 of FIGURE2 and illustrating the manual reset mechanism in depressed position,such depression having restored the thermal element to its initialposition of concavity from the position shown in FIGURE 4,

FIGURE 6 is a longitudinal sectional view of the device taken on theline 6-6 of FIGURE 2 and illustrating the mounting of the thermal discand contacts and terminal prongs within the device,

FEGURE 7 is a longitudinal sectional view taken on the line 7-7 ofFIGURE 2 showing the mounting of the heater element and the electricalconnections thereto within the interior of the housing of the device,and

FIGURE 8 is an end elevational view taken on the line 8-8 of FIGURE 1.

Referring more particularly to the drawings there is illustrated adevice embodying the present invention which takes the form of anelectrical plug for insertion into a conventional wall receptacle. Amolded housing for the plug, broadly indicated by the numeral 10contains the switching assembly for controlling the circuit through theplug and is preferably formed of a suitable electrical insulatingmaterial which is heat resistant to a fair degree such as a phenoliccondensation product or the like. EX- tending from one end of thehousing 10 are two prongs 11 and 12 and grounding pin 13 arranged toengage within a standard wall outlet, and from the opposite end aconnecting cord 14 having three insulated conductors 15, 16 and 17, asshown in FIGURE 1.

The plug housing it) is of substantially rectangular shape and comprisestwo mounting parts, one of which may be termed a hollow base member 18and the other a hollow cover member 119. In assembled relation the base13 and cover 19 define a housing chamber 26 having opposed longitudinalside walls and end walls. One end wall is formed with a cylindricalopening 21 therethrough to receive the connecting cord 14 and the otherend wall is arranged with three rectangular slots 22, 23 and 24extending therethrough for receiving the prongs 11 and 12 and groundingpin 13, respectively, projecting outwardly therefrom.

The cover member 19 is secured to the base member 18 by machine bolts 25and 26 extending through opposed aligned circular openings 27 and 23 inthe housing members l8 and 39 and which engage nuts (not shown) imbeddedin the outer surface of the base member 18. The cover member 1% hasopposed side walls 29 and 30 (FIG- URE 6) preferably molded to provide apleasing outer apperance. The side Walls 2h and 3d and end Walls 31 and32 define a rectangular cavity 34, within the cover for receiving andpositioning the elements of the switch.

Located in the top surface 33 of the cover member 19 (FIGURE 1) is acircular recess 35 for receiving a manually actuated reset button as, tobe described hereinafter. Surrounding the recess 35 is an upstandingcylindrical wall 37 extending from the top surface of the cover 19 asshown in FIGURES 1 and 2 and serving to protect the reset button 36 frombeing accidentally depressed. Extending from the base of the recess 35and concentric therewith is a circular recess 33 of smaller diameterwhich terminates in a bore 39 extending through the housing into thecavity 34 of the cover member as best shown in FIG- URE 2. Surroundingthe recess 3? and projecting upwardly into the recess 35 is a circularwall member til.

Projecting from the inner wall surface of the recess 35 and locatedwithin the rectangular cavity 34 of the cover member 19 is a circularwall 41 defining a cylindrical chamber 42 concentric with the circularrecess 35 in the top surface of the cover 19 and adapted to receive theswitching assembly hereinafter described. Integrally formed with thebase of the cylindrical chamber 4-2 are diametrically extendingupstanding wall members 43 which form opposed guadrants 44 and 45, theformer having openings 46 and 47 (FIGURE extending therethrough and thelatter an upstanding integrally formed post 48 (FIGURE 2). The opening4-6 receives the shank portion of a rivet 49 which has formed on theouter end thereof a fixed contact button 541?. Similarly, the opening 47receives the rivet 51 having formed on its outer end the fixed contactbutton 52. The circular wall 41 of the chamber 42 is provided with tworectangular openings 53 and 54 at approximately 90 apart for receivingcurrent carrying members also forming parts of the switch assembly. Theswitch chamber 42 is closed at its lower end by a removable cup-shapedclosure member 55 which fits snugly within the wall 41 of the chamber42. Formed integrally with the wall 41 and extending outwardly therefromis a wall member 56 which terminates in a rectangular supporting post 57upon which the inner end of the grounding pin 13 rests. A circular bore57a is formed in the top thereof to receive the shank of the terminalscrew 58 thereaded through the end of the grounding pin 13 for attachingthe ground lead thereto.

The base member 18 has longitudinal wall-s 59 and dtl and end walls 61and 62 forming a rectangular cavity 63 as shown in FIGURE 7. Located atthe line cord end of the cavity 63 is a rectangular recess d4 (FIGURE 2)for receiving the end of a ground lead 17 which is attached to thegrounding pin 13 by a terminal screw 58. Extending from the recess 64 tothe end wall 61 of the base member 18 there is formed a rectangularchamber 65 for receiving the grounding pin 13.

One insulated conductor 15 from the line cord 14 is attached by aterminal screw 66 to the angulated end of the terminal prong 11 withinthe housing 18. As shown in FIGURE 6 the terminal prong 11 is positionedwithin the cover member 19 and extends from its angulated inner endalong the inner surface of the side wall 29 and is bent at right anglesthereto along the inner surface of end wall 32 and then reversely bentand projected outwardly through the slot 23. The prong 11 is retained inposition by a raised shoulder portion formed on the end wall 61 of thebase member 18 which engages within the open end of the slot 23.

The other insulated conductor 16 of the line cord 14 is secured by aterminal screw 67 to an L shaped conductor arm 68 which is mounted onthe cover member 19 by means of a suitable rivet 69. The terminal screw67 extends through a threaded boss 70 in one leg of the L shaped arm 68and the shank end thereof may be received in a cylindrical recess 71formed in the cover member. The other leg of the L shaped conductor arm68 extends through the opening 53 in the circular wall 41 and terminatesin the switch chamber 42. Secured to the leg of the conductor arm withinthe switch chamber 42 is one end of a heating element 72 formed of asuitable electric resistance material which becomes heated to a desiredtemperature when a current overload is experienced in the circuit. Theheater element may be bent into any desired shape and is of a lengthsufficient to provide the desired resistance. I have found that theelement 72 should be formed of a material of the proper gauge andcomposition to provide the desired related overload time and temperaturecharacteristic fer the desired results. Usually an overload conditionmust exist for a period of time before enough heat will be generated tocause damage to the windings in an electrical motor. Therefore theheater element is selected which will generate sufiicient heat to beeffective on the switch elements to break the circuit before this timehas elapsed. On the other hand, nuisance tripping caused byinstantaneous current surges is to be avoided. The other end of theheating element 72 is secured to the leg 73 of a sheet metal strip 74,which leg projects into the switch chamber 42. The heating element 72 issituated within the chamber 55 as defined by the cup-shaped closuremember 55. The sheet metal strip '74 is fastened to the closure member55 of the switch chamber 42 by tabs 75 which are wrapped around the rim76 of the closure member. The metal strip 74 engages the upstanding leg77 of an angulated conductor member 78 and provides circuit continuitytherewith. The leg 77 of the conductor member '78 is positioned againstthe wall of the cylindrical chamber 4-2 as shown in FIGURE 5 and theother leg extends inwardly from the wall and is secured in position bythe rivet 49 below the contact button 5t) formed on the end thereof.

The rivet 51 carrying the contact button 52 anchors in position thelaterally extending end 79 of the prong 12 which passes through theopening 54 in the circular wall 41. The prong 12 engages the side wall3t) of the cover member 19 to the end wall 32 where it bends at rightangles thereto along the inner surface of end wall 32 and is reverselybent and projected through the slot 22. Similar to prong 11, prong 12 isretained in position within the slot 22 by a raised shoulder portionformed on the end wall 61 which engages within the open. end of the slot22.

Mounted within the cylindrical chamber 42 is a switching assemblyarranged to provide a thermal actuated breaking of the circuit when apredetermined temperature is reached by reason of a current overloadcausing heating of the element 72 to generate an excessive amount ofheat. Extending through the circular opening 39 at the center of therecess 38 is an internally threaded tubular shaft 8t having a circularflange which bears against the margin of the opening 39 on the innersurface of the cover member 19. The shaft fit) is slidably mounted inthe opening to be capable of occasional movement inwardly into thecylindrical switch chamber 42 within selected limits. The shaft 89threadedly receives an adjustable mounting screw 81 which extends beyondthe flanged end thereof and has a shouldered end forming mounting post81.

The post 81 serves to provide a center mount for a snap actingthermo-sensitive, bi-metallic disc 32 having two positions of concavityand which is so positioned in the cylindrical switch chamber 42 as tosnap from one position of concavity to the second position in responseto a rise in temperature. The disc %2 is of a proper size, compositionand configuration to provide the desired thermal responsivecharacteristics. This will depend to a great extent upon the particularheater element used as the relative reaction of these two members willdetermine the time delay between the commencement of the overloadcondition and breaking of the circuit. Such a delay is necessary toprevent nuisance tripping caused by instantaneous current surges in thecircuit. A circular opening in the center of disc $2 receives the post81 and the end thereof is riveted over to fasten the disc thereto. Thedisc 32 is formed with an opening 32a which receives the end of post 48,the latter serving to guide the disc in its movement within the switchchamber.

ttached to the bottom face of the disc 82 in diametric relation aremobile contact elements 83 and M, which are arranged to engage thestationary contacts 5t? and 52;, respectively, when the disc is in itsswitch closed position. Under current overload condition. however, thedisc becomes heated to a predetermined temperature by the heatingelement '72 and snaps to its second position of concavity thus breakingthe circuit as shown in FIGURE 4. The reset button 36 is positioned inthe circular recess 35 in the cover member 1% as best shown in FIG- URES1 and 2 and has a circular opening centered therein which receives theupper end of the tubular shaft 8%. Bearing against the lower surface ofthe reset button 36 is a helical spring 85 which is positioned aroundthe shaft and rests against the bottom wall of the circular recess 38.The button 36 is secured to the tubular shaft 3%) by a retaining ring 86which is pressed on the end of the shaft. A reset button cover plate 87fits on the top of the button and conceals the shaft and retaining ring.The button is thus spring loaded in an upward position but may bemanually depressed to push the bi-metallic disc through the center intoits switch closed position of concavity.

To attach the plug to the end of a line cord, the unit may bedisassembled by loosening the screws 25 and 26 and removing the cover.The conductors l5? and 16 from the line cord are then suitably looped attheir ends and placed around the loosened terminal screws 66 and 67 withthe ground lead 17 attached to screw 58 so as to be in engagement withthe grounding pin member 15. The terminal screws may then be tightenedto secure the conductors in position. The cover member 19 and basemember 18 are reassembled and the device is ready for use.

In the operation of the device, in the event of a current overload, theelement 72 would provide resistance to current flow sufiicient togenerate a rise in temperature which would heat the cylindrical chamber42 to the critical temperature of the bi-metallic element. This seriesof events would not occur instantaneously but would require definiteperiods of time so that a temporary overload would not cause the circuitto be broken. After a suitable time delay, the heat radiated by theelement causes the bi-metallic disc 82 to snap to its switch openposition as shown in FIGURE 4. Once the bimetallic disc has snapped tothis open switch position, it will remain so even though the temperatureof the element returns to normal since the forces in the element areinsuflicient to snap it over center under normal temperature conditions.

Once the element has reached normal temperature, however, thebi-metallic disc may be manually snapped back into its switch closedposition by pressing the reset button 36. This causes the disc to beforced downward at it center as shown in FIGURE 5. In so doing, theouter edge of the disc is brought into engagement with the wall of theclosure member 55' and further pressure on the central portion of thedisc moves it through center, and the disc is caused to snap back to itsinitial position of concavity. When the button 36 is released, themobile contact buttons 83 and 84 are engaged with the fixed contacts Stland 52, respectively, and the switch is closed.

It will be noted, however, that if the element '72 has not cooled downsufficiently when the reset button 36 is released the bi-metallic discwill immediately return to its switch-open position of concavity.Release of the button 36 will therefore not close the switch. Thisfeature prevents closing the circuit when an overloaded conditioncontinues to exist in the line.

The choice of bi-metal used for the disc 82 and the temperature setapplied thereto are critical factors in providing a device having thedesired current overload time and temperature responsivecharacteristics. Also, the choice of wire gauge, length and compositionfor the heater element 72 is important so that the delay in theoperation of the switch element is effective to preclude nuisancetripping of the circuit under instantaneous overloads or line surges andso that the device is effective only when the line is subjected to anoverload of suflicient duration to threaten to cause damage to the motorwindings of the electrical appliance or tool with which the plug isused.

It is to be understood that in accordance with the patent statutes,variations and modifications of the specific device herein shown anddescribed may be made without departing from the spirit of theinvention.

Having described my invention, I clam:

it. An electric plug for attaching a line cord from a small hand powertool or other electrical appliance to an electric power outletcomprising: a rectangular base member a cover member therefor fittingtogether to form an elongated plug housing of electric insulatingmaterial defining a cavity, a pair of prongs of conductive metal mountedin said housing on opposite longitudinal sides of said cavity havingportions projecting outwardly generally parallel to the oppositelongitudinal sides of said housing, said housing having slots at one endto receive said prongs and a line-cord-receiving opening at its oppositeend, a pair of laterally spaced line-cord-receiving terminals mountedwithin said housing adjacent said linecord-receiving opening, wallportions within said cavity defining a circular chamber having a lateralopening and a longitudinal opening connecting said chamber with saidcavity, one of said terminals being electrically connected to one ofsaid prongs, means for carrying an electric current through the other ofsaid prongs to the other of said terminals including switch meansconnected within said chamber and having a stationary contact and amobile contact and means for opening said switch means including abi-metallic disc operably connected to said mobile contact to move thesame into or out of contact with said stationary contact and an electricheating element mounted in said chamber adjacent said disc and receivingan electric current from one of said terminals, said disc having twostable positions of opposite concavity and holding the contacts togetherto close the circuit through said current carrying means when in thefirst of said stable positions and moving the contacts out of contactwhen heated suiriciently to snap to the other stable position, said discbeing mounted with its central axis generally parallel to thelongitudinal side walls of the plug and generally perpendicular to thebase and cover members of said plug housing, and means for electricallyconnecting said switch means with said other prong through said lateralopening and with said other terminal through said longitudinal openingand a spring-loaded manual reset button engaging the central portion ofsaid disc and projecting out of the cover member of said housing formoving said disc to said first stable position.

2. An electric plug for attaching a line cord from the motor of a smallelectric appliance to an electric power outlet comprising: a rectangularbase member and a cover member therefor fitting together to form a thinelongated plug housing of electric insulating material defining aninternal cavity and narrow side walls, a pair of spaced prongs ofconductive metal mounted in said housing and having end portionsprojecting outwardly from one end of the housing generally parallel tosaid side walls, said housing having a line-cord-receiving opening atthe opposite end of the housing, a pair of laterally spacedlinecord-receiving terminals mounted within said housing adjacent saidline-cord-receiving opening and electrically connected to said prongs,wall portions within said cavity defining a chamber having a lateralopening and a longitudinal opening connecting said chamber with saidcavity switch means in said chamber for making and breaking theelectrical circuit through the plug comprising a pair of fixed contacts,a pair of movable contacts, and a snap acting bi-metallic discsupporting said movable contacts at its opposite sides and conductingcurrent between said movable contacts when the switch means is closed,an electric heating element mounted in said housing adjacent said discto receive current from one of said terminals, said disc having twostable positions of opposite concavity and being mounted in said chamberbetween said side walls generally parallel to the base and cover membersof said housing, said disc holding the movable contacts against saidfixed contacts to close the said electrical circuit when in the first ofsaid stable positions and moving the contacts out of engagement to breaksaid electrical circuit when heated sufl'lciently to cause it to snap tothe other stable position, means for electrically connecting said switchmeans with one of said prongs through said lateral opening and with oneof said terminals through said longitudinal opening, means comprising amanual reset button connected to the central portion of said disc andprojecting through the cover member of said plug housing for moving saiddisc to said first stable position in a direction to close said switchmeans, and means biasing said reset button and said disc in a directionto close said switch means.

3. An electric plug for attaching a line cord from the motor of a smallpower tool or other electrical appliance to a conventional electricpower outlet comprising: a thin generally rectangular base membertherefor and a thin generally rectangular cover member fitting togetherto form an elongated plug housing of electric insulating material with alength corresponding to the length of said members, said housingdefining a cavity with two prongreceiving openings at one end and aline-cord receiving opening at the opposite end, a pair of prongs ofconductive metal mounted in fixed positions in said housing on oppositesides of said cavity and projecting outwardly through saidprong-receiving openings generally parallel to the sides of saidhousing, a pair of laterally spaced line-cord receiving terminalsmounted in said cavity adjacent said line-cord receiving opening, wallportions within said cavity defining a chamber having a lateral openingand a longitudinal opening connecting said chamber with said cavity, oneof said terminals being electrically connected to one of said prongs,means for carrying an electric current through the other of said prongsto the other of said terminals including switch means mounted withinsaid chamber and having stationary contacts and movable contacts andmeans for opening said switch means comprising a generally circularbi-metallic disc connected to said movable contacts to move the sameinto or out of contact with said stationary contacts and an electricheating element mounted in said chamber adjacent said disc to receive anelectric current from one of said terminals, said disc having two stablepositions of opposite concavity and holding the movable contacts againstsaid stationary contacts to close the circuit through said currentcarrying means when in the first of said stable positions and moving thecontacts out of engagement to break said circuit through said currentcarrying means when heated sufliciently to cause it to snap to the otherstable position, said disc being mounted in said chamber generallyparallel to the cover member, and means for electrically connecting saidswitch means with other of said prongs through said lateral opening andwith other of said terminals through said longitudinal opening, a fixedstop mounted in said cavity on one side of said disc for engaging themarginal portion of said disc, and a manual reset button mounted on saidcover member and connected to the central portion of said bimetallicdisc on the side thereof opposite said stop for pressing said discagainst said stop in a direction to open said switch means to return thedisc to said first stable position, said reset button projecting throughthe said cover member.

An electric plug for attaching a line cord from the motor of a smallpower tool or other electrical appliance to a conventional electricpower outlet comprising: a thin generally rectangular base member and athin generally rectangular cover member therefor fitting together toform an elongated plug housing of electric insulating material with alength corresponding to the length of said members, said housingdefining a cavity with two prong-receiving openings at one end and aline-cord receiving opening at the opposite end, a pair of prongs ofconductive metal mounted in fixed positions in said housing on oppositesides of said cavity and projecting outwardly through saidprong-receiving openings generally parallel to the sides of saidelongated plug housing, a pair of laterally spaced line-cord receivingterminals mounted in said cavity adjacent said line-cord receivingopening, wall portions within said cavity defining a chamber having alateral opening and a longitudinal opening connecting said chamber withsaid cavity, one of said terminals being electrically connected to oneof said prongs, current carrying means for connecting the other of saidterminals in series with the other of said prongs comprising switchmeans mounted in said chamber between said cover memher and said basemember and having a pair or" stationary contacts, a pair of mobilecontact buttons, and a snapacting bi-rnetallic disc supporting saidmobile contact buttons at diametrically opposed portions of the disc, anelectric heating element mounted in said chamber adjaent said disc toreceive an electric current from one of said terminals, said disc havingtwo stable positions of opposite concavity and holding the mobilecontact buttons against said stationary contacts to close the circuitthrough said current carrying means when in the first of said stablepositions and moving the contacts out of engagement to break saidcircuit through said current carrying means when heated sufficiently tocause it to snap to the other stable position, said disc being mountedin said chamber generally parallel to the base member and cover member,means for electrically connecting said switch means with said other ofsaid prongs through said lateral opening and with said other of saidterminals through said longitudinal opening, a fixed stop mounted insaid chamber on one side of said disc for engaging the marginal portionof said disc, and a manual reset button connected to the central portionof said bi-metallic disc on the side thereof opposite said stop forpressing said disc against said stop in a direction to open the switchmeans to return the disc to said first stable position, said resetbutton projecting through the said cover member.

References Cited by the Examiner UNITED STATES PATENTS 2,199,307 5/40Bolesky 200-122 X 2,427,390 9/47 Dear 200-136 2,783,329 2/57 Jackson 2002,839,638 6/58 Epstein 200-113 2,911,503 11/59 Garbcrs 200122 BERNARD A.GILHEANY, Primar 1 Examiner.

ROBERT K. SCHAEPER, Examiner.

1. AN ELECTRIC PLUG FOR ATTACHING A LINE CORD FROM A SMALL HAND TOOL OROTHER ELECTRICAL APPLIANCE TO AN ELECTRIC POWER OUTLET COMPRISING: ARECTANGULAR BASE MEMBER AND A COVER MEMBER THEREFOR FITTING TOGETHER TOFORM AN ELONGATED PLUG HOUSING OF ELECTRIC INSULATING MATERIAL DEFININGA CAVITY, A PAIR OF PRONGS OF CONDUCTIVE METAL MOUNTED IN SAID HOUSINGON OPPOSITE LONGITUDINAL SIDES OF SAID CAVITY HAVING PORTIONS PROJECTINGOUTWARDLY GENERALLY PARALLEL TO THE OPPOSITE LONGITUDINAL SIDES OF SAIDHOUSING, SAID HOUSING HAVING SLOTS AT ONE END TO RECEIVE SAID PRONGS ANDA LINE-CORD-RECEIVING OPENING AT ITS OPPOSITE END, A PAIR OF LATERALLYSPACED LINE-CORD-RECEIVING TERMINALS MOUNTED WITHIN SAID HOUSINGADJACENT SAID LINECORD-RECEIVING OPENING, WALL PORTIONS WITHIN SAIDCAVITY DEFINING A CIRCULAR CHAMBER HAVING A LATERAL OPENING AND ALONGITUDINAL OPENING CONNECTING SAID CHAMBER WITH SAID CAVITY, ONE OFSAID TERMINALS BEING ELECTRICALLY CONNECTED TO ONE OF SAID PRONGS, MEANSFOR CARRYING AN ELECTRIC CURRENT THROUGH THE OTHER OF SAID PRONGS TO THEOTHER OF SAID TERMINALS INCLUDING SWITCH MEANS CONNECTED WITHIN SAIDCHAMBER AND HAVING A STATIONARY CONTACT AND A MOBILE CONTACT AND MEANSFOR OPENING SAID SWITCH MEANS